I'd picked up Phineas Redux at a charity shop and was about to read it when I read somewhere that it was important to read Phineas Finn first.

Reading Phineas Finn, I came across the following speech:

"...Many who before regarded legislation on the subject as chimerical, will now fancy that it is only dangerous, or perhaps not more than difficult. And so in time it will come to be looked on as among the things possible, then among the things probable;—and so at last it will be ranged in the list of those few measures which the country requires as being absolutely needed. That is the way in which public opinion is made."

Which struck me as a good way of describing the Overton Window.